In some Low Emission Zones, converting the vehicle to an alternative fuel will allow entry into the LEZ.

Fuels that can be retrofitted can include:

This conversion may also enable the vehicle to have a higher Euro standard, as you would when replacing the engine with a newer diesel or petrol engine. Gas conversions often also have to be certified and you will need to provide some proof to the LEZ authorities.

Different countries have different approaches to proof of certified conversions. In London you need to register with the Vehicle Standards Authority (VOSA), see the TfL website. In other countries you may be able to have the change registered on your vehicle papers, or provide other proof for the LEZ authorities. Check with the relevant low emission zone authorities before undertaking the conversion.

In many countries alternative fuels can be cheaper (due to reduced fuel tax), although there can be significant costs to the conversion itself.

Ethanol or Biogas conversions are more widely used in Sweden than elsewhere. 

Due to their zero emissions while driving, electric and hydrogen vehicles are also generally allowed in LEZs as well as other access regulations. However, these are more likely to be new vehicles than converted vehicles.